Plasma and Fusion Research: Rapid Communications

Volume 10, 1202082 (2015)

First Measurement of Electron Temperature and Density Profiles
for Spherical Tokamak Plasmas Sustained by Lower Hybrid Waves
Hiro TOGASHI, Akira EJIRI, Hiroto HOMMA, Takahiro SHINYA, Yuichi TAKASE,
Kazuya TOIDA, Naoto TSUJII, Takashi YAMAGUCHI, Yusuke YOSHIDA,
Makoto HASEGAWA1) , Yoshihiko NAGASHIMA1) , Hirokazu FURUI, Kenta NAKAMURA,Wataru TAKAHASHI, Toshihiro TAKEUCHI, Masateru SONEHARA,Satoru YAJIMA and Hibiki YAMAZAKIThe University of Tokyo, Kashiwa 277-8561, Japan1)
Kyushu University, Kasuga 816-8580, Japan(Received 29 June 2015 / Accepted 1 September 2015)
In this research, the profiles of electron temperature T e and density ne for a spherical tokamak with theplasma current sustained by lower hybrid waves alone have been measured for the first time using Thomson
scattering diagnostics in the TST-2 spherical tokamak device. The T e profile was hollow and the ne profile waslike bell-shaped. T e and ne near the plasma center were 6 eV and 6 × 1017 m−3 , respectively, leading to a pressureof 0.6 Pa. On the other hand, whole pressure at the plasma center calculated using an equilibrium reconstruction
code EFIT was around 20 Pa. Therefore, it is suggested that fast electrons play an important role in the plasmaequilibrium.
c 2015 The Japan Society of Plasma Science and Nuclear Fusion Research

Keywords: TST-2, spherical tokamak, Thomson scattering, lower hybrid wave, current drive
DOI: 10.1585/pfr.10.1202082
In order to realize a compact spherical tokamak (ST)reactor, which has no space for a central solenoid, a plasma
current needs to be started-up non-inductively. Plasma current start-up experiments using electron cyclotron wave
(ECW) have been performed in several ST devices [1–4].
On the other hand, the lower hybrid wave (LHW) is usedin the TST-2 [5]. A successful lower hybrid current drive(LHCD) was achieved with a high eﬃciency in many conventional tokamaks, but the eﬃciency is low for ST devices [6, 7]. For LHCD optimization in STs, it is important to obtain electron temperature T e and density ne profiles, leading to understanding of wave propagation and absorption. T e and ne for plasmas sustained by the LHW
were measured in Globus-M [7], but the profiles were notreported. Thomson scattering (TS) diagnostics is widelyused to measure the profiles. However, TS measurement
becomes diﬃcult when electron density is low becauseof the weak TS signal. Plasmas started-up by radiofrequency (RF) generally have lower density than those forohmically started-up plasmas by tens to hundreds times.
In order to improve the signal-to-noise ratio in such lowdensity plasmas, several techniques have been adopted, including averaging many raw TS signals over reproducible
discharges [8], and a multi-pass scheme wherein a laser
pulse makes many round trips in an optical cavity [9, 10].
TST-2 is a spherical tokamak with major radius R =0.36 m and minor radius a = 0.23 m. Typical discharges

in the present experiment are shown in Fig. 1. A 2 kW
ECW is injected for plasma initiation, and the plasma current is started-up and sustained by a LHW power of 20 kW.
The plasma current Ip is approximately 5 kA, and the line-Fig. 1 Waveforms of (a) plasma currents, (b) radiation measured
by an absolute extreme ultraviolet (AXUV) detector, (c)LCFSs, and (d) LHW and ECW injected powers for 5 discharges. Part (f) shows a typical line-integrated density at
the midplane.

author’s e-mail: togashi@fusion.k.u-tokyo.ac.jp

1202082-1

c 2015 The Japan Society of Plasma
Science and Nuclear Fusion Research

Plasma and Fusion Research: Rapid Communications

Volume 10, 1202082 (2015)

Fig. 2 TS signals and the fittings at R = 342 mm. Blue font values represent the center wavelength of the transmission
band of each interference filter in the polychromator.

Fig. 3 Profiles for (a) electron temperature T e and (b) electron
density ne with errors for each are plotted as red opensquares. Purple lines represent limiters on both sides.
The solid curve in the Fig. (b) shows the Gaussian fit.

integrated density ne l at the midplane measured by interferometry is approximately 1.3 × 1018 m−2 (l ∼ 0.45 m).
Good reproducibility can be seen in the traces of Ip s andradiation signals. For the TS measurements, we use an Nd:
YAG laser with wavelength 1064 nm, energy 1.6 J, repetition rate 10 Hz, and width 10 ns. The collection system has
a scattering angle of 120◦ near the plasma center, a solid
angle of 0.27 sr, fibers with a core diameter of 2 mm, anda numerical aperture of 0.37. The detection circuit for thepolychromator is optimized so that relative errors become
small [11]. In the present experiment, 10 TS measurements
for 4 spatial points were performed simultaneously, and the10 TS signals for each spatial point were averaged to attainhigher accuracy. Similar measurements were performedfor 4 other spatial points, and then profiles with 8 spatial
points were obtained. Laser injection time was t = 85 ms,
when the plasmas were stable.
Figure 2 shows TS signals at R = 342 mm obtained by averaging over 10 TS signals and subtracting
the background. The signals of 6 wavelength channels
in each polychromator were fitted well into a relativistic
Maxwellian distribution function. Resultant profiles forT e and ne are shown in Fig. 3. Note that one of the spatial points (R = 459 mm) is significantly aﬀected by straylight and the results are not shown in Fig. 3. T e values in
the central region are under 10 eV and the profile is hollow. The ne profile is like bell-shaped with a peak nearR = 342 mm. The relative sensitivity of the ne measure-ment was obtained by Raman scattering from nitrogen gas.
The absolute value of ne is calibrated by comparing thevalue of ne l measured by interferometry with ne l calculated from the ne profile measured in an ohmic discharge,
where the whole density profile was obtained accurately.
The peak density was ne ∼ 6 × 1017 m−3 and we fitted
the data into a Gaussian shape (solid curve in Fig. 3 (b)).
The line-integrated density for this shape agrees with ne lmeasured by interferometry. In addition, ne was less than1 × 1016 m−3 at the outer limiter according to probe measurements. Thus, the fitted Gaussian shape seems to bereasonable, although the density in the peripheral region
was not measured.
The shape of the T e profile obtained is similar to thatreported in QUEST, where the plasmas are sustained by
ECW power alone [8]. Although ECWs and LHWs are different, the similar hollow T e profiles may be caused by acommon physical phenomenon in RF-sustained ST plasmas.
The central electron pressure for the bulk part of thevelocity distribution function obtained by the TS measurement was under 1 Pa. On the other hand, whole plasmapressure including the tail part is calculated from magnetic
measurements using an equilibrium reconstruction code
EFIT to be around 20 Pa near the plasma center. Thus,
it is suggested that the pressure of fast electrons generated by LHWs is dominant in the equilibrium. In addition,
the eﬀective temperature for fast electron estimated fromhard X-ray signals is in the order of 10 keV, and the ion
temperature T i is determined to be several eV from spectroscopy [12]. Taking the temperatures and ni  ne into
account, the density for the fast electrons is in the order of1015 m−3 and the abundance ratio is in the order of 0.1%.
In summary, electron temperature and density profiles
in ST plasmas sustained by LHWs have been measured
successfully for the first time, helping us understand LHWpropagation and absorption in ST plasmas.
This work was supported by Japan Society for the Promotion of Science (JSPS) Grant-in-Aid for Scientific Research No. 21226021 and by the National Institute for Fusion Science (NIFS) Collaborative Research Program Nos.
NIFS14KOCR001 and NIFS12KUTR078.
[1] H. Meyer et al., Nucl. Fusion 53, 104008 (2013).
[2] T. Maekawa et al., Nucl. Fusion 45, 1439 (2005).
[3] K. Hanada et al., Plasma Fusion Res. 5, S1007 (2010).
[4] A. Ejiri et al., Nucl. Fusion 46, 709 (2006).
[5] T. Shinya et al., Nucl. Fusion 55, 073003 (2015).
[6] T. Wakatsuki et al., AIP Conf. Proc. 1580, 422 (2014).
[7] V.V. D’yachenko et al., Plasma Phys. Rep. 39, 219 (2013).
[8] T. Yamaguchi et al., Plasma Fusion Res. 8, 1302001(2013).
[9] M. Yu. Kantor et al., Plasma Phys. Control. Fusion 51,055002 (2009).
[10] H. Togashi et al., Rev. Sci. Instrum. 85, 056103 (2014).
[11] A. Ejiri et al., Plasma Fusion Res. 5, S2082 (2010).
[12] S. Tsuda et al., Plasma Fusion Res. 10, 1202064 (2015).

1202082-2